54
While I was playing Pearl last night I decided to check the Pokemon Day Care. It turns out I left a Bidoof and a Kricketot there a few years back and completely forgot about them. Both were very early catches of mine that I had trouble training as I moved forward into the game (Kricketot especially, since it only knows one damaging move that takes up three turns to charge and release, and more often than not, it misses, even on the weakest wild Pokemon if I recall right) but nonetheless I decided to pay $6800 to get them both back.
...they are now level 39 and 41, respectively.